Production of diphenyl



M. DEGEoRGEs ET AL 3,112,349

Nov. 26, 1963 PRODUCTION OF DIPHENYI..

2 sheets-sneu 1 Filed July 19. 1961 Nov. 26, 1963 M. E. DEGEORGES ET A1. 3,112,349

PRODUCTION OF DIPHENYL 2 Sheets-Sheet 2 Filed July 19, 1961 FIG. 2.

, MARCEL E. DE'GEORGES MA UR/CE JA YMOND INVENTORS BY%;H Il 'sv j TTORNEY United States Patent O 3,112,349 PRODUCTION F DIPHENYL Marcel E. Degeorges and Maurice Jaymond, Lyon,

France, assignors to Societe Progil, Paris, France, a

corporation `of France Filed July 19, 1961, Ser. No. 125,192 Claims priority, application France Dec. 6, 1960 Claims. (Cl. 260-670) This invention relates to the production of diphenyl and more particularly to a method and process for the chemical conversion of benzene to diphenyl.

The chemical diphenyl is an important industrial substance and is used as a heat exchange medium as well as an intermediary in the manufacture of sulfonated phenyls. Although there are several methods of making diphenyl, the important commercial process involves the pyrolysis of benzene vapors. When the pyrolysis method is used, the raw benzene vapor is passed through tubes with heat applied to their exterior, the heat passing through the wall of each tube into the benzene in the interior whereby the chemical process takes place. The major problem in the pyrolysis of benzene vapor is that carbon forms on the inner surface of the tubes. This carbon effectively limits the length of time that the apparatus may be used before the process must be stopped and the tubes cleaned. First of all, the carbon restricts the flow of material through the tube and, secondly, it acts as a thermal insulation which prevents the heat from passing properly through the tube wall into the flowing chemical. Another problem encountered in the known variations of the pyrolysis process is that various other polyphenyls are produced at the same time as diphenyl; the presence of these undesirable higher phenyls causes the etiiciency of the process to be low and presents problems of separation. These and other diiculties experienced with the prior art processes have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide for the production of diphenyl by a process which can be carried on for an extremely long period of time without interruption and which operates with high eiiiciency.

Another object of this invention is the provision of a process for producing diphenyl in which the deposit of carbon on the walls of the apparatus is substantially reduced.

A further object of the present invention is the provision of a method of producing diphenyl by the pyrolysis of benzene, wherein very little carbon is deposited on the process tubes and the method can be carrying on for extremly long periods of time without shut-down for cleaning of the tubes.

It is another object of the instant invention to provide a process for producing diphenyl by pyrolysis in which the simultaneous production of polyphenyls is limited.

It is a further object of the invention to provide a process for the production of diphenyl by pyrolyzing benzene whereby production may be continued on an industrial scale without interruption for long periods of time and wherein the hourly production of diphenyl is considerably increased with good yield and in a state of satisfactory purity.

With these and other objects in View, as will be apparent to those skilled in the art, the invention resides in the cornbination of steps set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to certain apparatus by which the process is carried on, as illustrated by the accompanying drawings in which:

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where 2r is the inner diameter of the pyrolysis tube, v is the velocity of the vapor, d is the specific weight o-f the vapor at the temperature of the reaction, and n is the viscosity of the vapor at this temperature. Best results take place when the process `is carried on according to the invention wit-h turbulences corresponding to Reynolds numbers ranging between 120,000 and 500,000. The pyrolysis is carried on at the same temperature as it has in the past, that is to say, from 550 C. to 900 C. and, more particularly, from 600 C. to 850 C. However, in contrast to conventional processes, 4the present process involves heating the benzene vapor to temperatures which progressively increase from the inlet to the outlet of the reaction zone. According to the conventional technique, the benzene vapor is rst preheated to the particular pyrolysis temperature which is necessary for the time required to convert from 1% to 30% of the benzene to the diphenyl. However, `according to the preferred embodiment of the present invention, it is advantageous to introduce the benzene vapor Iinto the pyrolysis zone While it is still at a temperature too low for a substantial conversion to take place; then, the benzene vapor lis heated within the pyrolysis Zone itself up to a temperature high enough for a rapid formation of diphenyl.

According to the present process, there is only one reaction zone instead of the two zones used in conventional practice, the conventional process being carried on with a preheating zone and a reaction Zone; with a single zone the control of the conversion becomes easier and its thermal yield -is improved.

According to a preferred version of the process, the benzene vapor has a temperature o-f from 450 C. to 700 C. at the inlet of the reaction zone and it leaves this zone at from 700 C. to 870 C. For instance, the reaction zone can be fed with benzene vapor at 550 C. and the vapor may leave the reaction zone at 870 C. Best results, however, are obtained when the temperature ranges of from 450 C. to 650 C. at the inlet and 750 C. to 800 C. at the outlet of the pyrolysis reaction zone. Thus, it can be seen that the new process involves establishing a temperature gradient of from 100 C. to

200 C. from the beginning to the end of the pyrolysis zone.

In order t-o produce diphenyl in industrial quantities so that the turbulences with Reynolds numbers higher than 100,000 can play a completely beneficial role in the process, it is desirable adequately to control the length of time which the benzene vapor takes to pass through the they involve low thermic yields and involve the necessity of using special materials for the construction of the reaction tubes. Of course, the actual time for the pyrolysiis may vary, depending upon the range of temperatures used. As a practical matter, in the regular production of large amounts of diphenyl, it is preferable that the time of pyrolysis of the vapor should lie between 0.4 and 1.5 seconds.

A certain inter-dependence exists between the length of time of the process and the particular turbulence or Reynolds number used. In fact, in a reaction zone of given dimensions the length of time of the pyrolization determines the linear velocity of the vapor. IIndeed, the velocity is precisely one of the factors which determine the degree of turbulence. In the formula for the Reynolds number, the value of d 'and n being constant at a given temperature, the Reynolds number depends only upon the velocity v and the inner diameter 2r of the pyrolysis zone. Therefore, it is possible to have the same turbulence in the range of Reynolds numbers between 100,000 and 700,000 with various reaction tubes having different diameters, 2r. After establishing for each of the tubes the velocity, the product of 2r and v remains constant; with pyrolysis times varying from 0.1 to 3.0 seconds, the vapor velocities may vary in the ratio of 30: 1. Of course, the same Reynolds number may be obtained in tubes having inner diameters which vary from 1 to 30. However, the present invention aims at a limited range of diameters which permit carrying out the invention with particularly advantageous results.

An apparatus for carrying out the process of the invention is illustrated in the drawings. Referring tirst to FIG. 1, which best shows the general features of an apparatus for practising the process of the invention, a pyrolysis apparatus, indicated generally by the reference numeral 10, is shown as having an entrance pipe 11 leading into a storage tank I12. The bottom of the storage tank is connected by a pipe 13 to one side of a valve 14, the other side of which is connected to one end of a pipe 15 extending through a vaporizer 16. The vaporizer is provided with a heating coil 17 connected to a source of heating steam (not shown). It is also provided with another coil 18 whose use will be described further hereinafter. A pipe 19 connects the pipe 15 of the vaporizer 16 to a tube 22 of a tubular heat exchanger 21; the other end of the tube is connected to a pyrolysis furnace 23 having a tube 24 which will be described in greater detail later on. The entrance end of this tube is connected to the tube 22 and the other end of which is connected through a pipe 25 to a tube 26 lying within the heat exchanger 21.

The other end of the tube 26 is connected by a pipe 27 to one end of the coil 18 which lies in the vaporizer 16. The other end of the coil 18 is connected by a pipe 28 4to the lower end of a fractionating column 29. The central portion of the 4fractionating column 14 is connected to a reiiux regulator valve 31. The upper part of the fractionating column 29 is connected to the upper part of a condenser 32, the bottom of which is connected to a cylinder 33, the bottom of which is connected by a pipe 34 to the storage tank 12. The top of the cylinder 33 is connected to `a cooler 35 having a top exit pipe 36 from which hydrogen can be removed. The bottom of the fractionating column 29 passes into the top of a second column 37 having a heating coil 38. Any benzene remaining is distilled off through a pipe 39 leading back to the first column 29. The bottom of the column 37 is connected by a pipe 41 to a third column 42 having a heating coil 43 at its lower portion. A lreiiux coil 44 is located at the upper part of the column 42 :and is connected through a valve 45 to a water storage container 46. The water which may be evaporated in `the coil 44 passes through a pipe 47 to a condenser 4S, the bottom of which is connected by a pipe 49 to the top of the water storage tank 46. The top of the column 42 is connected by a pipe 51 to a coil 52 lying in a vessel 53. The coil 52 is completely immersed in a body 54 of boiling water, the vapor from which is condensed by a cooling coil 55 located in the upper part of the vessel 53. Finally, pure diphenyl leaves the bottom of the coil 52 through a pipe 56 which is connected to a storage drum 57. Similarly, the bottom of the column 42 is connected by a pipe 58 -to a drum 59 in which the polyphenyls are stored.

The pyrolysis furnace 23 is shown in detail in FIG. 2. The furnace is provided with a vertical tubular steel wall 61 which is lined with insulating material 62 in which are embedded electrical heating elements 63. Lying within the housing defined by the wall 61 and the insulating material 62 is the coil 24 which is arranged in a series of convolutions lying on an imaginary cylinder coaxial with the |wall 61. The entrance pipe 22 is connected to the coil as is the exit pipe 25. Each convolution consists of a U-shaped bend 64 which merges into vertical straight portions 65.

rThe inside diameter of the tube 24 may be selected in the range between 55 and 205 mm. One of the factors which must be taken into account in selecting the diameter is the desired hourly production of diphenyl. However, suitable diversification of production may be obtained with a pyrolysis tube having a diameter in the ran-ge between mm. and 140 mm. with best results in the range of from to 120 mm. With tubes having too great a diameter, heat exchange becomes difficult, while, when the tubes are selected of too small a diameter, the difficulties described above in connection with the prior art processes are encountered. The length of the tube is, of course, determined by the exact process and the diameter of the tube. However, experience shows that industrial production is greatly facilitated when the tubes have a length of from 30 to 100 meters and more particularly in the range of from 45 to 75 meters. Preferably, the tubes are made of refractory steel.

As is evident in FIG. 2, the tube is formed into a plurality of bends along its length; however, it is advisable that the tube have a bend at each portion in the range of from 2 to 10 meters in length with preference being given to the range of from 3 to 5 meters. In contrast to the conventional helical coils which are often used in the art, the bends of the tube 24 have their radius of curvature as little as possible compatible with the diameter of the tube. It is preferable that the radius of the axis of the tube in the bend to be of the same order as the external diameter of the tube; #good results are obtained when the radii equal 1 or 2 times the outside diameter. At the same time, is is important that the straight parts 65 form an angle to one another as close as possible to 0, and in any case, form an angle of less than 30. With the radius of curvature selected in the manner shown, the distance between two successive straight portions 65 will be in the same order as the outside diameter of the tube.

The operation of the process of the invention will now be readily understood in the light of the above description. Liquid benzene is fed through the pipe 11 into the storage .tank 12 and from the tank the benzene passes into the vaporizer 16 through the pipe 13, the flow being controlled by the valve 14. The interior of the vaporizer is heated by steam passing through the 'heating coil 17; the benzene passing through the pipe 15 is converted into la vapor which leaves the vaporizer 16 through the pipe 19. 'Ihe vapor reaches the tubular heat exchanger 21 where its temperature is raised by the absorption of heat from the vapors arriving at the heat exchanger through the pipe 25 and residing within the tube 26. Benzene vapor then passes through the pipe 22 into the pyrolysis furnace 23. After passing through the coil 24 in which the reaction takes place, the pyrolyzed vapors leaving the furnace by the pipe 25 now contain non-:transformed benzene, completely manufactured diphenyl, polyphenyls, and hydrogen. rlhese elements then pass through the tube 26 in the heat exchanger 21 and through the pipe 27 into the coil `18 within the vaporizer 16. There heat is transferred :to the fresh benzene in the tube as it cornes from the storage tank 12. These vapors are conducted by `the pipe 28 to the bottom of the first column 29 in which the benzene is separated from the pyrolysate. Benzene distilled in the column 29 is liquified in the condenser 32. The central region of the column 29 is kept at constant temperature by means of the reliux regulator valve 31. From the condenser 32 liquid benzene flows into the intermediate cylinder 33 from which it returns to the storage tank 12. So far as the hydrogen evolved is concerned, it is cooled to a low temperature within the coler 35 in order to recover any benzene it carries before it is taken away at the top through the pipe 36.

Crude diphenyl which still contains polyphenyls and also a little benzene is the liquid fraction recovered at the bottom ofthe column 219 and this is permitted fto run into the second column 37 having the heating coil 3S in its lower portion. From the column 37 the remainder of the benzene distillls off to the column 29 while a mixture of diphenyl and polyphenyls enter the third distillation column 4t2. The-se two elements are then separated. The heat required for the separation is provided by the heating coil 43 at the bottom of the column. Adequate reflux is obtained at the top of the column 42 by circulating water through the coil 44, the inlet of which is provided with the valve 45. Water vaporized within the coil 44 liquilies in the condenser `48 from which it returns through the pipe 49 lto the storage container 46, the llower part of which is connected with the coil 44- through the valve 4S. Purified diphenyl vapor which leaves the top of the column 42 is condensed to the liquid state within the coil 5'2 in the vessel 53. This coil is situated in the bottom part of the 'vessel 53 which contains the body 54 of boiling water sucient for the coil 52 to be completely immersed. As the water boils, its vapor is condensed in the upper coil 55 within which cold water is circulated. In this way, the condensation of pure diphenyl is formed without any crystallization of this compound within the coil 52. Finally, the pure liquid diphenyl which flows down from the coil 26 through the pipe 56 is cooled and crystallized in the drum 57, while the polyphenyls separated at the bottom of the column 42 pass through the pipe 58 into the drum 59 where they are solidified..

Now, returning to the passage of the benzene vapors through the coil 24 ofthe :furnace 23, it will be understood that heating of the substance takes place because of the electrical heating element 63. As has been lstated above, the diameter of the tube has been related to the temperatures and so on of the benzene vapor according to the formula for the Reynolds number in such a way that the Reynolds number of the flow through the tube is in the range of from 100,000 to 700,000. These coils are, of course, connected to an electrical source (not shown). The yfollowing are `several examples illustrating the electiveness of the process of the present invention:

Example l In an installation similar to that shown in FIG. l, 3,000 kilograms of diphenyl Were produced continuously per 24 hours. The reaction tube was similar to those shown in FIG. 2. The tub-e was 50 meters long and had 11 successive bends, so that there were l2 straight portions of about 4.08 meters each. The internal diameter of the `tube was 115 mm. and the radius of curvature of the bends at the axis of the tube was 120 mm. The distance between the axes of two adjacent straight portions was about 235 mm. The temperature of the benzene vapor entering the apparatus through the pipe 22 was 560 C. and the heating by means of coil 63 was so controlled that the vapor left the apparatus through the pipe at a temperature of 800 C. The tube was continuously fed with 1390 kilograms per hour of benzene vapor. The flowing vapor remained within the tube 24 for 1.32 seconds, which means that the vapor flowed with a linear velocity of 38.4 meters per second. 'The average Reynolds number of the vapor, therefore, was 080,000. With each passage :of the vapor through -the pyrolysis tube '9% of the benzene was converted into diphenyl with a yield of 92%. 'Ihere was practically no yformation of tar or carbon within the reaction tube and the tube Worked uninterruptedly for ten months. Diphenyl was produced at the rate of 240 g. per hour per liter of capacity of the react-ion tube.

Example II In a process lcarried on in the manner similar to that of Example I, the pyrolysis tube was 67 meters long instead of 50 meters. The benzene vapor entered through the pipe 22 at 600 C. and left the tube 24 through the pipe 25 at a temperature of 750 C. 'Ihe time of its passage through the pyrolysis zone was 1 second and the Reynolds number was 320,000. During each passage through the pyrolysis zone, 10% of the benzene was transformed into diphenyl with a yield of 87% and there were no carbon deposits. 4.5 tons of diphenyl was thus produced every 24 hours and it was possible Ito vary the daily produtcion between 1.5 and 5 tons with the same reaction tube. The above results should be compared with tests carried out according to the prior art at the same average temperature of 675 C. with a pyrolysis time or 1 second within a tube having internal diameter of 25 mm. and a length of 8 meters; in this case the Reynolds number was 4,000 and only 5.5% of the benzene was converted into diphenyl in each passage through the pyrolysis tube.

Example III tube, 10% of the benzene was transformed into diphenyl with a yield of The continuous process was not interrupted by carbon deposits in the 'tube nor by tar formation. It is worthy of note that the reaction tube used produced 360 grams of diphenyl per hour per liter of its capacity :compared with the prior art processes in which there were never more than 85 grams per hour per liter produced, Example IV With the appara-tus used in Example I equipped, however, with a reaction tube having an internal diameter of 115 mm. and a length of 50 meters, 4.7 tons of diphenyl were produced steadily every 24 hour period. The temperature was 675 C. at the inlet of the tube and 800 C. at its outlet. Vapor remained within the pyrolysis zone for 0.45 second and its turbulence amounted to a Reynolds number of 470,000. At each passage 5% of the benzene was transformed into diphenyl with a yield of 95%. The hourly production of diphenyl per liter of capacity of the pyrolysis tube amounted to 375 grams. I'here is no indication of carbon deposit.

Example V t Diphenyl was produced in a reaction tube the internal kdiameter of which was mm. and the length 35 mete-rs.

Example VI The production of diphenyl was effected within a pyrolysis tube 96 meters long having an internal diameter of 200 mm. The benzene vapor entered the tube at 460 C. and left it at 710 C., the mean temperature being 585 C. The vapor passed through the tube in 2.7 seconds with what corresponded to a Reynolds number of 352,000. The conversion of benzene in the diphenyl was 4.5% with a yield of 96% and there was no indication of carbon deposit.

Example VII The same tube used in Example VI was used at a constant temperature of 700 C. with the ow through the tube taking place in 1.17 seconds and a Reynolds number of 680,000. Wtih each passage of the vapor through the tube 3.4% of tlhe benzene was converted Iinto diphenyl giving a yield of 96.5%. The daily production of diphenyl was 7.5 tons which meant tha-t 105 grams of diphenyl per hour per liter of capacity of the reaction tube took place. There was no indication of carbon deposit.

Example VIII In this case the pyrolysis tube was 50 meters long and 77 mm. diameter. yIt was used with a temperature of 550 C. at the inlet and 800 C. at the outlet, this being a mean temperature of 680 C. The benzene vapor remained within the tube for 1.32 seconds (as in Example I) and the Reynolds number was 120,000. The conversion of benzene into diphenyl was 8.8% with each pass and a yield of 92%. Daily production was 1.33 tons which meant that 236 grams per hour per liter of the capacity of the tube took place. No carbon was deposited on the inside surfaces of the tube.

The following is a chart comparing the operation and results of the prior art processes with the results obtained with the process carried on according to the present invention:

Prior art Examples of the application A B C 1 2 3 4 I. Inlet temperature, C 700 675 700 560 600 G30 675 II. Outlet temperaturc, C 700 675 700 800 750 770 800 III. Mean temperatime1 700 675 700 680 675 700 737 IV. Internal diameter 2r of reaction tube in 0111---- 2. 5 2. 5 5.3 11.5 11.5 l1. 5 11.5 V. Linear velocity ot vapor in cm./sec- 21. 5 92. 7 225 3, 840 6, 700 7, 900 11,100 VI. Length of reaotion tubeinmeters- 7. 5 7. 5 4.00 50 67 67 50 VII. Capacity of reaction tubeln liters. 3. 7 3. 7 8.8 518 694 694 518 VIII. Percent of benzene converted into diphenyl per passage 23.8 6.6 3.3 9 10 10 5 IX. Yield on benzene, percent 66. 5 92 94 92 87 85 95 X. Time of passage of vapor through reaction tube, seoonds 34.8 8.1 1. 76 1.32 l 0.85 0.45 XI. Reynolds Number 11,020 1, 050 4, 580 180, 000 320, 000 360, 000 470, 000 XII. Dipheny1produced in grams per hour per liter of reaction tube capacity 85.5 33 (i. 7 246 313 360 375 1 Pressure, 3.55/kg/cm.2

It can be seen, then, that one of the advantages of the present process is that good results can be obtained under normal pressure, while in the prior art conversion rates more than 7% could be obtained only under elevated pressure (see col-umn A of the table). It should be understood that -in industrial practice a certain amount of head is required for causing the benzene vapor to pass through the pyrolysis tube. The head would be equivalent to a column of benzene of about 5 to 20 meters high. A head of 13 meters of benzene corresponds to a pressure of 1.2 kgs. per square centimeter. As a practical matter, in general a pressure is used of from 0.4 to 1.4 kgs. per square centimeter and more often than not 0.9 to 1.2 kgs. per square centimeter are used in addition to the atmospheric pressure at the inlet in the pipe 22 of the pyrolysis tube 24. Of course, there is a certain loss of head within the tube, depending on the form and size of the latter. For example, when two tons of diphenyl are produced at 770 C. within a tube 76 meters long and mm. in diameter, with 18 bends, the loss of head is from 0.4 to 0.5 kg. per square centimeter.

The ligures in the tables show that the percentage conversion of benzene into diphenyl per second of pyrolysis time is much greater in the process of the invention than in the prior art (compare the horizontal lines VIII and X). In fact, when one divides the figures in line VIII by those of -line X it is seen that the percent of benzene converted per second within the pyrolysis tube are, according to the prior art, (a) 0.685, (b) 0.815 and (c) 1.865 while, according to the invention, Example I gives 6.8, Example II gives 10.0, Example III gives 11.8 and Example IV gives 11.1. The table also shows that the production of diphenyl per hour per liter of capacity of the reaction tube is considerably increased (see line XII).

The advantages of the invention with regard to eicency and good yield appears very clearly when we compare the present process with one of the best method-s previously known described in U.S. Patent No. 2,702,307. When the process of the said patent is carried out, there is a yield of about 93% diphenyl with respect to benzene for each passage of the benzene vapor through the pyrolyzing zone, while the rate of conversion of benzene is 5%. Now, with the present invention, the conversion rate of benzene is from 9 to 10% which is about twice the conversion rate. Moreover, the production of diphenyl per liter of the capacity of the reactor used per second is l0 times that which can be obtained according to the above-mentioned patent. Another comparison that can be made would be comparing the present process with the process described in U.S. Patent No. 2,099,350; when the process described in that patent is used, the tube within which the pyrolysis reaction is carried out must be replaced at least every 8 to 10 days because of the carbon deposits formed on the inner wall of the tubo. In comparison, when the present process is used with the pyroly'sis tube described above, the process can work uninterruptedly as long as several months with each single tube daily producing 5 to 10 tons if diphenyl.

The benefits of the present invention result from the fact that the production of diphenyl is improved when the benzene vapor passing through the reaction zone is subjected to intense turbulence, much stronger than that which generally prevailed the prior art reaction tubes. The usefulness of a certain degree of turbulence is well known. For example, in U.S. Patent No. 1,907,498, which issued in 1933, the application of this knowledge led to passing the vapor rapidly through the pyrolyzing tube, but velocity of the vapor was limited by the fact that the conversion rate in each passage decreased as the velocity increased. At that time -a turbulence corresponding to a Reynolds number of 100,000 was considered as the upper admissible limit, but in practice, operations were carried out at much lower tunbulences, say in the order of 600 to 5,000 Reynolds number.

It is obvious that minor changes may be made in the details of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact procedure herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new `and desired to secure by Letters Patent is:

1. A method of producing diphenyl by the pyrolysis of benzene vapor, comprising the steps of passing the vapor through 1an elongated pyrolizing zone having a diameter in the range from 55 mm. to 205 mm., applying heat to the Zone, and causing the vapor to pass through the zone with a turbulence having a Reynolds number in the range `from 120,000 to 500,000.

2. A method as recited in claim 1 wherein the vapor remains within the pyro-lization Zone for a period of time in the range from 1.1 to 3.0 seconds.

3. A method as recited in claim 1 in which the teinperature of the vapor is increased by an amount in the range from 100 C. to 200 C. while the vapor passes through the Zone.

4. A method as recited in claim 1 wherein the length of the zone lies in the range from 30 to 100 meters.

5. A method as recited in claim 1 wherein the vapor is introduced int-o one end of the zone at a temperature in the range from 550 C. to 650 C. and the vapor leaves the other end of the tube at a temperature in the range from 750 C. to 800 C.

6. A method as recited in claim 1 wherein the vapor is caused to pass through the tube in `a period of time in the range from 0.4 to 1.5 seconds.

7. A method as recited in claim 1 wherein the vapor passes through the zone `and is heated to a temperature in the range from 500 C. to 900 C.

8. A method Yas recited in claim 1 wherein the length of the zione lies in `the range from 45 to 75 meters.

9. A method as recited in claim 1 wherein the vapor is introduced into one end of the tube `at a temperature in the range from 550 C. to 650 C. and the vapor leaves the other end of the tube at a temperature in the range from 750 C. to 800 C., the temperature of the vapor Ibeing increased by an amount in the 4range from 100 C. to 200 C. while the Vapor passes through the heated zone.

l0. A method of producing diphenyl by the pyrolysis lof benzene vapor, comprising the steps of passing the vapor through an elongated pyrolizing Zone having an inside diameter or the zone being in the range from mm. to 205 mm., `applying heat Vto the exterior of the tube, introducing the vapor into one end of the tube at `a temperature in the range from 450 C. to 700 C., causing the vapor to leave the other end of the tube at a temperature in the range from 700 C. to 870 C., causing the vapor to pass through the tube in a period of time in the range from 0.1 to 3.0 seconds, so that the flow through the tube takes place with turbulence having `a Reynolds number in the range from 120,000 to 500,000.

References Cited in the le of this patent UNITED STATES PATENTS 1,907,498 Carothers May 9, 1933 2,009,350 Stoesser Nov. 16, 1937 2,702,307 Saunders et al Feb. 19, 1955 

1. A METHOD OF PRODUCING DIPHENYL BY THE PYROLYSIS OF BENZENE VAPOR, COMPRISING THE STEPS OF PASSING THE VAPOR THROUGH AN ELONGATED PYROLIZING ZONE HAVING A DIAMETER IN THE RANGE FROM 55 MM. TO 205 MM., APPLYING HEAT TO THE ZONE, AND CAUSING THE VAPOR TO PASS THROUGH THE ZONE WITH A TURBULENCE HAVING A REYNOLDS NUMBER IN THE RANGE FROM 120,000 TO 500,000. 